1. Field of the Invention
The present invention relates to a method of cleaning exhaust gas. It relates further to an apparatus for cleaning the exhaust gas implementing such method.
2. Description of the Prior Art Art
Drying plants discharge in addition to dust and water vapor depending on the prevailing matter being dried also water vapor volatile, gaseous, organic substances together with the exhaust gas being produced and, furthermore, depending on the prevailing matter being dried also flue ash.
During the drying of wood chips in wood chip board plants, for instance, directly heated rotary dryers are used. In these dryers the moisture present in the matter to be dried is reduced from 30-120% to 2-5%. This exhaust gas contains in addition to solid matter and water vapor, a complex mixture of inorganic and organic non-air matter or substances, respectively.
Noteworthy is now the fact, that the organic substances are at least partly volatile, natural constituents of wood but, however, also further substances which are produced during the reducing of the wood substance during the processing thereof in chip board production plants. With regard to the quantity the most significant such substances are the terpenes, which are natural constituents of wood. It has been found, furthermore, that carbon acids, formaldehyde, methanol, phenol and cresols are contained in the exhaust gas.
A common procedure has been to clean or purify, respectively, such exhaust gas in cyclones, by means of which mean dust values of 100-200 mg/Nm.sup.3 have been reached in the so-called purified gas, whereby, however, peak values can occur in a magnitude of 1000 mg/Nm.sup.3. The exhaust gas temperature prevailing thereby amounts generally to 110.degree.-130.degree. C. and the content of water vapor 300-350 g/Nm.sup.3. A separating and removing of the above mentioned organic and gaseous constituents has hardly been made until this date. Accordingly, the exhaust gas was still loaded with considerable amounts of dust and of organic, gaseous substances, which are termed in the art "blue smoke" as well as a considerable content of water vapor which is termed in the art as "white smoke".